The invention is in the field of optical waveguides, such as optical fibers, used to detect and track changes in wavelength of the light in the waveguide. It also relates to optical fibers which have a fiber Bragg grating.
Systems are known in which a signal input into an optical waveguide such as an optical fiber will undergo changes of its wavelength. The changes can be characterized as slow changes and rapid changes in wavelength. There are many causes of these changes, depending upon the particular system.
An example of such a system is one which uses a fiber Bragg grating (FBG) as a strain or temperature measuring device. An FBG is useful because it converts changes of a physical parameter such as strain or temperature to wavelength changes. A detection system can be used to subsequently convert the wavelength changes to an electrical signal, which can be employed for further processing. In such systems, for example in the strain gauge case where the subject under test undergoes strain, the output of the FBG varies over time due to the strain effects exhibiting the slow wavelength changes noted above. On the example of a temperature detection system, temperature changes also will typically be reflected as slow type changes in the FBG spectral response. The subject under test may undergo rapid change such as from vibration which will result in the FBG output reflecting the vibration as rapid changes in wavelength.
In some cases both slow and rapid changes may occur. It is desirable to be able to track the changes and to measure the changes in wavelength.
Typically such systems generate a light signal in a band of wavelengths about a central wavelength. While the wavelength of the light signal is representative of some measured or sensed parameters or varying event, the range of wavelength shift, in particular of the central wavelength, may be of such magnitude that the sensitivity of the measuring apparatus is reduced or even lost.
Thus, a problem occurs when the central wavelength of the incoming signal, or the wavelength range of interest moves out of the range where the detection system features its highest sensitivity.
This invention relates to processing signals in an optical waveguide which have slow or rapid changes in wavelength or both, and to apparatus and method for tracking the slow changes. The invention further relates to use of an optical fiber having a tunable Fiber Bragg Grating (tFBG) to determine the changes in wavelength as part of a closed feedback loop.
In accordance with the principles of the invention, the tFBG transmits light in a band of wavelengths about center wavelength characteristics of the tFBG. The closed loop path operates to determine the instantaneous maximum intensity of the light signal output from the tFBG. A processor in the closed loop produces a voltage (Vsignal) which corresponds to the instantaneous maximum intensity and compares the voltage, in real time, to a preset voltage which corresponds to the maximum intensity when the center wavelength of the tFBG matches the center wavelength of the optical signal (Vset). The difference between Vsignal and Vset is applied to a mechanism for physically changing the tFBG to adjust Vsignal toward being equal to Vset.
In further accordance with the principles of this invention, an optical apparatus and method for measuring the intensity of light waves, representative of some measured parameter, includes a feedback loop operative to adjust the center wavelength of the band of wavelengths to which a Fiber Bragg Grating (FBG) responds.
The feedback loop includes a tFBG which is coupled to a processor operative to convert the light energy (intensity) of the input light signal into an electrical signal. The amplitude of the electrical signal corresponds to the instantaneous intensity of the, light signal. The feedback loop thus is operative to determine the maximum instantaneous amplitude (intensity) and to maintain the center wavelength of the tFBG at a value which corresponds to the instantaneous maximum intensity of the incoming light signal.
The processor in the feedback loop determines the maximum intensity value, a priori, and by assigning a voltage value Vset, representing a test or calculated maximum sensitivity value for the characteristic central wavelength of the tFBG. The instantaneous intensity value (Vsignal) is compared to that assigned value (Vset) on a real time basis and the tunable Fiber Bragg Grating is adjusted to move its center wavelength to (ideally) equal that of the assigned value, Vset.
In further accordance with the principles of the invention, there is described an apparatus and a method for processing an optical signal in an optical waveguide using wavelength-dependent transmission characteristics. The signal is in an optical waveguide, such as an optical fiber, the change in wavelength of which is to be tracked, is passed into a tFBG. A closed loop feedback system using the tFBG maintains the central wavelength of the transmitted signal from the tFBG at a peak wavelength operating point to compensate for large changes of the processed signal.
The output of the tFBG is coupled to an optical processor to convert the light energy into an electrical signal proportional to the light intensity variations which are a function of the wavelength changes. That electrical signal is then compared to a second electrical signal which is set to a selected predetermined level which represents the central wavelength of the optical signal. The comparison determines the difference between the central wavelength of the signal exiting the tFBG converted to an electrical signal and the preset electrical signal. Then, the tFBG is physically adjusted to reduce that difference to a minimum. By tuning the tFBG so that the output signal of the tFBG and the preset signal are equal, or toward minimizing their difference, in a feedback loop, the tFBG tracks and maintains the change in wavelength of the incoming signal within a range of high sensitivity of the tFBG. In addition to actively tracking and adjusting the central wavelength, the change can be read for use.
The varying instantaneous output of the detector is designated as Vsignal and the selected preset electrical signal is designated Vset. The feedback loop seeks to establish Vsignal combined with Vset to equal zero, that is, the feedback loop is operative to eliminate the difference between Vsignal and Vset. A controller does this by changing its output signal according to the difference and then operating a tuning apparatus to tune the tFBG. That is, the tuning apparatus changes the central wavelength of the signal through the tFBG and therefore Vsignal in a direction towards being equal to Vset.
In a preferred embodiment of the invention the optical signal source which provides an input or incoming optical signal, also referred to as a subject optical signal, is a sensor such as a temperature or strain sensor. An optical fiber connected between a sensor and a tunable FBG provides a subject optical signal to the tunable FBG. The sensor or other source of the incoming optical signal may also be a FBG. The sensor may be a plurality of sensors in which case a wavelength division multiplexer may be employed to distinguish the signal from each of the sensors. The plurality of sensors may comprise a plurality of FGBs.